* More generic comparison code from docker_container to docker_common.
* More flexibility if a is None and method is allow_to_present.
Note that this odes not affect docker_container, as there a is never None.
* Update docker_secret and docker_config: simplify labels comparison.
* Added unit tests.
* Use proper subsequence test for allow_more_present for lists.
Note that this does not affect existing code in docker_container, since lists
don't use allow_more_present. Using allow_more_present will only be possible
in Ansible 2.8.
* pep8
